This invention relates to rechargeable or secondary batteries and, in particular, to rechargeable or secondary batteries using zinc negative electrodes.
Various types of zinc secondary batteries are known in the art. Typical batteries are Ni--Zn, Ag--Zn, Zn--O.sub.2 and MnO.sub.2 --Zn.
Zinc secondary batteries employ zinc negative electrodes which exhibit a decay or reduction in capacity with the repetitive charge-discharge cycling of the battery. As a zinc secondary battery and, therefore, each of its zinc negative electrodes, is cycled, the zinc active material of each electrode becomes redistributed over the surface of the electrode. This redistribution of active material or so-called "shape change" of each zinc electrode is a result of the solubility of the zinc active material in the alkaline electrolyte of the battery and causes a reduction in the capacity of the battery.
The solubility of the zinc active material in the battery electrolyte also causes an increase in the battery gas pressure due to the decreased availability of the zinc active material to recombine with oxygen generated during charging. It similarly leads to an increase in the rate of hydrogen evolution by the battery. Accordingly, a rapid buildup of gas pressure occurs in the battery, often requiring that the battery be vented. Constant venting of the battery, however, causes loss of water through electrolysis. This, in turn, causes dryout of the battery electrodes, reducing battery life.
Over the years, many different zinc negative electrodes have been proposed to reduce electrode shape change. One such zinc negative electrode is disclosed in U.S. Pat. No. 3,516,862 issued to W. Van der Grinten. In the '862 Patent, Ca(OH).sub.2 is added to the zinc active material (ZnO) to reduce the solubility of the active material through the formation of CaZn.sub.2 (OH).sub.6 (calcium zincate). However, in these electrodes, the formed calcium zincate experiences thermodynamic instability due to dissociation in the battery electrolyte.
Other sealed zinc batteries have been proposed to improve gas recombination in the sealed battery. In these batteries, the shape change in the zinc electrode is still a concern.
It is, therefore, an object of the present invention to provide an improved zinc negative electrode and zinc secondary battery.
It is a further object of the present invention to provide a zinc negative electrode and zinc secondary battery with reduced shape change and solubility of the zinc electrode and increased cycle life for the battery.
It is a yet further object of the present invention to provide a zinc negative electrode and zinc secondary battery meeting the above objectives and having improved gas recombination, permitting the battery to be sealed and maintenance free.